The invention relates to distributed process control systems, more particularly to distributed process control systems which are integrated to perform sequential and continuous control and data acquisition.
Process control technology involves the application of control techniques to industrial processes to permit achievement of a desired level of performance and desired results. The historical development of process control technology reflects the evolution of a wide variety of control techniques to solve particular process control problems. However, the core issue in the solution of process control problems centers on the availability of tools with which to provide these solutions. The evolution of control techniques has in large part been based on the evolutionary development of control tools.
The industrial control business of the 1950's was divided into two different groups, the first being sequential control which used relays to provide motor control, and the second being continuous or modulating process control which used pneumatic devices to sense and control valves and pumps. Because the apparatus, i.e. motors, valves and pumps, for which the control tools were developed was manufactured by independent manufacturing industries, the control techniques evolved independently and there was no need for any integration.
With the solid state revolution came the ability to provide control tools based upon electronic circuits. The 1960's saw logic directors replace motor control relays, and also saw electronic analog devices replacing pneumatic modulating instruments. Even with control tools based on similar technology, sequential and continuous control systems remained independent along the lines of the supplier apparatus manufacturing industries. This period also saw the introduction of plant computers for data acquisition and monitoring, although not performing control functions.
At this stage, the goal of total process management appeared on the horizon. As plant computers grew in processing capability, it became evident that they could also absorb control responsibilities. Thus, the evolution of the direct digital control (DDC) computer system permitting integration of control and data acquisition functions, with the added capability of total process optimization and management. For a while, this seemed to be the ultimate solution for process control. However, application experience proved otherwise. A combination of factors was at work, notably the tendency to funnel all process control functions into a given piece of equipment, the plant computer. System response times became intolerable. Added to this was the difficulty of coordinating the motor control and instrumentation engineering design groups responsible for different portions of a given plant. Thirdly, this approach gave birth to the industry expression "the system is down" which meant that an entire plant control system was inoperable, often with disastrous results.
The middle 1970's brought about radical changes in the approach to sequential and continuous process control. The introduction of programmable controllers (PC's) allowed sequential control techniques to be programmed in the form of familiar relay ladder diagrams which were representative of the sequentially controlled process. These PC's also allowed continuous process control functions, traditionally analog in nature, to be performed in digital fashion. At this stage, process control was divided into essentially independent segments, each segment executing in a low cost microprocessor. Distribution of different control functions among many independent processing units eliminated the sluggishness and reliability problems of the DDC system. A serial process control data highway allowed an operator to communicate with the process via a color CRT screen. The data acquisition functions were kept in a separate minicomputer, and a data port allowed transfer of separately gathered process data onto the data highway.
This stage of evolution presented three essentially independent systems, a distributed system for sequential control, a distributed system for continuous process control, and a central computer system for data acquisition and optimization. Each had been designed independently, and none had been designed with the idea of integration in mind. When used for primarily one type of control, each of these systems was successful. It is important to note that at this stage of evolution, three different programming languages and programming techniques were required to implement these systems. This cumbersome programming problem was compounded by the lack of simple and meaningful user documentation for all three independent systems. This is the stage of development from which most process control systems in use today are derived.
However, the evolution in hardware as described above was accompanied by an evolution in the approach to solution of process control problems. Entering the ranks of the control design engineers were those trained to approach process control system problems so as to maximize process efficiency. Therefore, their ability to apply ladder diagrams for sequential control and process flow diagrams for continuous process control had to be matched by available hardware in which to implement these two techniques. Because of the independent hardware development in each of these areas, the control design engineer was thwarted in his attempt at integration. In addition, the desirability of including data acquisition capability further compounded the problem because of the tendency for a designated plant computer to burden the system response time in accessing information along the data highway. In order to advance to the next stage of distributed process control, there had to be a means to enable data highway communication that would eliminate the system sluggishness and allow integration of sequential and continuous process control functions along with data acquisition in a single control system. Just such a significant development was the subject matter of the applications outlined in the cross-reference at the beginning of the present application. There, a data highway communication system for a distributed process control system was disclosed in great detail, indicating the advantage of having a global data base available for distributed processors along a data highway. The global data base contains all of the information necessary to permit sequential and continuous process control while also handling data acquisition functions. However, that series of applications, while alluding to the capability of performing sequential and continuous control and data acquisition functions in a single system, provided only a general description of the distributed processing unit or drop connected to the data highway and the method of programming it to perform these functions.
The preceding discussion of the evolution of distributed processing control systems points to the desirability of a distributed processing control unit or drop in such a system which is connected to a data highway and which is capable of performing sequential and continuous process control and data acquisition functions all in the same unit. Such a universal process control device would enable a control design engineer to distribute the various process control tasks among several such drops while integrating in each of them the capability for performing a combination of control techniques. It would also be desirable if such a distributed processing control unit were easily programmed through a single programming language and technique. It would also be desirable if such a programming technique provided the user with simplified documentation of the control system configuration.